Coronary heart disease (herein “CHD”) is the leading cause of death in many industrial countries. Atherosclerosis is a form of arteriosclerosis or hardening of the arteries in which there is the progressive build-up of plaque containing cholesterol and lipids in blood arteries. This build-up is associated with an increased risk of heart disease and morbid coronary events. The build-up of plaque in the arteries is associated with an immune response that is triggered by damage to the endothelium. Initially, monocyte-derived macrophages accumulate at the damaged site, due to the immune response causing a migration and accumulation of smooth muscle cells which form fibrous plaque in combination with the macrophages, lipids, cholesterol, calcium salts and collagen. The growth of such lesions can eventually block the artery and restrict blood flow.
Lipoprotein-associated phospholipase A2 (Lp-PLA2), also known as PAF acetylhydrolase, is a secreted, calcium-independent member of the growing phospholipase A2 superfamily (Tew, et al. (1996) Arterioscler Thromb Vasc Biol. 16(4):591-9; Tjoelker, et al. (1995) Nature 374(6522):549-53). It is produced by monocytes, macrophages, and lymphocytes and is found associated predominantly with LDL (˜80%) in human plasma. The enzyme cleaves polar phospholipids, including sn-2 ester of 1-O-alkyl-2-scetyl-sn-glycero-3-phosphocholine, otherwise known as platelet-activating factor (herein “PAF”) (Tjoelker, et al. (1995) Nature 374(6522):549-53).
Many observations have demonstrated a pro-inflammatory activity of oxidized LDL when compared with native unmodified lipoproteins. One of the earliest events in LDL oxidation is the hydrolysis of oxidatively modified phosphatidylcholine, generating substantial quantities of lysophosphatidylcholine (lyso-PC) and oxidized fatty acids. This hydrolysis is mediated solely by Lp-PLA2 (i.e., Lp-PLA2 hydrolyzes PAF to give lysophosphatidylcholine [“lyso-PC”] and acetate) (Stafforini, et al. (1997) J Biol. Chem. 272, 17895).
Lyso-PC is suspected to be a pro-inflammatory and pro-atherogenic mediator. In addition to being cytotoxic at higher concentrations, it is able to stimulate monocyte and T-lymphocyte chemotaxis, as well as induce adhesion molecule and inflammatory cytokine expression at more modest concentrations. Lyso-PC has also been identified as the component of oxidized LDL that is involved in the antigenicity of LDL, a feature that may also contribute to the inflammatory nature of atherosclerosis. Moreover, lyso-PC promotes macrophage proliferation and induces endothelial dysfunction in various arterial beds. The oxidized fatty acids that are liberated together with lyso-PC, are also monocyte chemoattractants and may also be involved in other biological activities such as cell signaling). Because both of these products of Lp-PLA2 hydrolysis are potent chemoattractants for circulating monocytes, Lp-PLA2 is thought to be responsible for the accumulation of cells loaded with cholesterol ester in the arteries, causing the characteristic “fatty streak” associated with the early stages of atherosclerosis.
Lp-PLA2 has also been found to be enriched in the highly atherogenic lipoprotein subfraction of small dense LDL, which is susceptible to oxidative modification. Moreover, enzyme levels are increased in patients with hyperlipidaemia, stroke, Type 1 and Type 2 diabetes mellitus, as well as in post-menopausal women. As such, plasma Lp-PLA2 levels tend to be elevated in those individuals who are considered to be at risk of developing accelerated atherosclerosis and clinical cardiovascular events. Thus, inhibition of the Lp-PLA2 enzyme would be expected to stop the build up of this fatty streak (by inhibition of the formation of lysophosphatidylcholine), and so be useful in the treatment of atherosclerosis. Furthermore, Lp-PLA2 can be used as a biomarker to determine if an animal is at risk for developing a disease associated with elevated Lp-PLA2 levels or elevated Lp-PLA2 activity.
Lp-PLA2 inhibitors inhibit LDL oxidation. Lp-PLA2 inhibitors may therefore have a general application in any disorder that involves lipid peroxidation in conjunction with the enzyme activity, for example in addition to conditions such as atherosclerosis and diabetes other conditions such as rheumatoid arthritis, stroke, myocardial infarction (Serebruany, et al. Cardiology. 90(2):127-30 (1998)); reperfusion injury and acute and chronic inflammation. In addition, Lp-PLA2 is currently being explored as a biomarker of coronary heart disease (Blankenberg, et al. J Lipid Res. 2003 May 1) and arteriosclerosis (Tselepis and Chapman. Atheroscler Suppl. 3(4):57-68 (2002)). Furthermore, Lp-PLA2 has been shown to play a role in the following disease: respiratory distress syndrome (Grissom, et al. Crit Care Med. 31(3):770-5 (2003); immunoglobulin A nephropathy (Yoon, et al. Clin Genet. 62(2):128-34 (2002); graft patency of femoropopliteal bypass (Unno, et al. Surgery 132(1):66-71(2002); oral inflammation (McManus and Pinckard. Crit Rev Oral Biol Med. 11(2):240-58 (2000)); airway inflammation and hyperreactivity (Henderson, et al. J Immunol. 15; 164(6):3360-7 (2000)); HIV and AIDS (Khovidhunkit, et al. Metabolism. 48(12):1524-31 (1999)); asthma (Satoh, et al. Am J Respir Crit Care Med. 159(3):974-9 (1999)); juvenile rheumatoid arthritis (Tselepis, et al. Arthritis Rheum. 42(2):373-83 (1999)); human middle ear effusions (Tsuji, et al. ORL J Otorhinolaryngol Relat Spec. 60(1):25-9 (1998)); schizophrenia (Bell, et al. Biochem Biophys Res Commun. 29; 241(3):630-59 (1997)); necrotizing enterocolitis development (Muguruma, et al. Adv Exp Med Biol. 407:379-82 (1997)); and ischemic bowel necrosis (Pediatr Res. 34(2):237-41 (1993)).
Lp-PLA2 activity from human samples has been measured using spectrophotometric activity and fluorogenic activity assays (Cayman Chemical Company, AtheroGenics, Inc. and Karlan Research Products). See also Kosaka, et al. Clin Chem Acta 296(1-2):151-61 (2000) and Kosaka, et al. Clin Chem Acta 312(1-2):179-83 (2001). However, these methods may be insensitive when inhibitor to Lp-PLA2 is present, particularly when the inhibitor is administered to an animal prior to obtaining a sample from the animal. The assay of the current invention has been shown to demonstrate a correlation between Lp-PLA2 inhibitor concentration in a sample and Lp-PLA2 activity. Lp-PLA2 activity measured over time in patients treated with inhibitor correlated with the pharmacokinetic profile of the inhibitor.
Radiolabeled PAF has been used in low throughput assays for Lp-PLA2 activity. Tselepis, et al. Arterioscler Thromb Vasc Biol. 15(10):1764-73 (1995) and Min, et al. Biochemistry, 40(15):4539-4549 (2001). However, these methods have not been developed as high throughput methods and therefore are not useful for large scale studies compared with the present invention. Lp-PLA2 concentration from human samples has been measured using an ELISA assay using high throughput methods. A strong correlation has been found between the current activity assays available and the mass or ELISA assay. However, the mass or ELISA assay is probably not sensitive to detecting Lp-PLA2 inhibitors in samples. In order to measure Lp-PLA2 activity with or without inhibitor in a large-scale study or to screen a plurality of samples for Lp-PLA2 as a selected biomarker, a high throughput activity protocol is required. Accordingly, a method for determining LP-PLA2 activity from a plurality of samples is greatly needed.